Cancer is a term used for diseases in which abnormal cells divide without control and are able to invade other tissues, spreading to other parts of the body through the blood and lymph systems. There are hundreds of different types of cancer, and in order to select the best treatment for an individual patient as much information as possible about the origin and characteristics of the cancer is required.

Most cancers are classified according to the organ in which they arise, for example breast or colon, and by the type of cell that is effected, for example ductal or lobular types of breast cancer. Haematological malignancies arise from cells that originate in the bone marrow, but because these cells are widely dispersed throughout the body this was not recognized until relatively recently.

Haematopoiesis (simplified)

Simplified diagram of haematopoiesis

Hence, historically haematological malignancies were divided according to the organs that appeared to be most involved, rather than the cell populations of origin. The three main groupings were leukaemias where the bulk of disease is in the blood, lymphomas where it is in the lymph nodes, and myeloma where it is in the bones. The lymphomas were further divided according to the pattern of lymph node involvement into non-Hodgkin lymphoma and Hodgkin lymphoma, which preferentially occurs in cervical (neck) nodes.

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Although the fourfold subdivision of haematological cancers into leukaemias, Hodgkin lymphomas, non-Hodgkin lymphomas and myelomas has no practical application, it is still commonly used by cancer registries to describe the burden of disease in the population as a whole. In the UK, for example, these groupings comprise around 9% of all newly diagnosed cancers – which is broadly similar to other economically developed regions of the world. However, we now know that such basic tallies do not include all haematological cancers; and that this classification has no clinical value.

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In 2001, the World Health Organization (WHO) produced a new consensus classification that defined haematological malignancies according to their cell lineage, genetic abnormalities, and clinical features. This classification, which was updated in 2008 and defines over 60 disease subtypes, was uniformly adopted into clinical practice and is incorporated into the International Classification of Diseases for Oncology (ICD-O-3). This classification is used within HMRN.

The distinction between lymphoid and myeloid cell types is fundamental to the modern classification. Like many organs, the bone marrow contains a small population of stem cells that persist throughout life. When these cells divide the daughter cells can remain as stem cells or begin a process of differentiation that leads to the production of functionally specialized cells. In the bone marrow the first step in this process is a commitment to the myeloid or lymphoid pathway. Cells in the myeloid pathway ultimately become red blood cells, platelets or a range of specialized cells that can recognize and destroy bacteria and some other types of microorganism. Cells in the lymphoid pathway become T-lymphocytes or B-lymphocytes, and these form the main components of the immune system.

Haematopoiesis (detailed)

Detailed diagram of haematopoiesis

All of these cells circulate between lymph nodes and other organs through the blood and lymphatic systems; some undergoing further differentiation during an immune response producing, for example, functionally specialized plasma cells that produce large quantities of antibodies.

Antibody classes

Diagram of antibody classes

The processes of cellular differentiation are highly complex, and there are many points along the pathway where cells can acquire the genetic abnormalities that could lead to cancer development. The cancers that arise bear the hallmark of their origin, and their unique features form the basis of the current WHO ICD-O-3 classification.

The most fundamental separation is the division between those of lymphoid origin, which account for around three-quarters of all haematological malignancies, and those of myeloid origin which account for the remainder. Within the lymphoid group, mature B-cell malignancies dominate, accounting for more than 90% of all lymphoid cancers.

Cell lineage

Myeloid Lymphoid split pie chart

In addition to being based on biological origins and features, WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues contains several cancers that were not recognized as such in the past. These include, for example, the chronic myeloproliferative neoplasms and myelodysplastic syndromes which comprise around two-thirds of all myeloid malignancies.

Myeloid Lymphoid annual rate

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